Ever since the invention of organic field effect transistors (OFETs) in the 1980s their performance could be continuously improved. Nowadays, OFETs are used for driving e-ink displays, printed RFID tags, and flexible electronics. The advantages of OFETs compared to silicon technology are the possibility to realize thin and flexible circuits at low process temperatures on large areas.
In general, an organic field effect transistor comprises a gate electrode, a source electrode and a drain electrode. Further, the OFET comprises an organic semiconductor and a gate insulator which separates the gate electrode from the organic semiconductor.
Despite the progress, the widespread application of OFETs is still limited due to their low performance and stability. However, there is a large potential for improvement by the development of advanced OFET structures.
Although the organic doping technology has been shown to be a key technology for highly efficient opto-electronic devices, the use of doped organic layers in organic transistors is much less common.
There are three different approaches to improve the performance of OFETs.
For example, doping can be used to reduce the contact resistance at the source and drain electrodes. A thin p- or n-doped layer between the metallic electrodes and the organic semiconductor forms an ohmic contact which increases the tunnel currents and enhances the injection of charge carriers.
Some groups reported on the effects of channel “doping” on the OFET performance. For example, it is possible to switch pentacene transistors from p-type to n-type using a monolayer of Ca at the oxide surface. The monolayer covers the surface of the insulating layer completely and acts as a local “pseudo-dopant”. There is no charge carrier transfer between Ca and the insulating layer. Instead, an electric field is generated by the Ca atoms in the monolayer. The Ca monolayer fills electron traps at the interface between the organic semiconductor and the gate insulator. It was shown that “doping” the channel of an n-OFET by an air-stable n-dopant can increase the air-stability of n-type transistors.
Furthermore, it has been reported that the threshold voltage can be shifted by the doping concentration. Meijer et al. Journal of Applied Physics, vol. 93, no, 8, p. 4831, 2003, studied the effect of doping by oxygen exposure on polymer transistors. Although a shift of the switch-on voltage (the Hatband voltage) was observed, the effect was not related to doping by the authors. Similarly, other authors found a similar shift of threshold voltage with applying a dopant, but often this effect is rather related to the influence of contact doping than to channel doping.
Inversion FETs are normally OFF and an inversion channel has to be formed by an applied gate voltage in order to switch the transistor ON. Inversion FETs are used in CMOS circuits and are the most basic building block of all integrated circuits. It is known that the inversion regime cannot be reached in organic MIS (metal insulator semiconductor) capacitors. However, it has been predicted by simulations that an inversion channel can be formed in FET structures, if minority carriers are injected at the source and drain electrodes. Huang et al. Journal of Applied Physics, vol. 100, no. 11, p. 114512, 2006, could show that a normally n-conducting intrinsic material can be made p-conductive by charging the gate insulator prior to deposition of the organic layer by a corona discharge.
Document US 2010/0096625 A1 discloses an organic field effect transistor comprising a substrate on which a source and a drain electrode are arranged. A semiconducting layer is deposited on top of the electrodes and in electrical contact with the electrodes. The semiconducting layer is formed with a lower sublayer and an upper sublayer. On top of the upper sublayer a dielectric layer and a gate electrode are provided. The semiconductor materials of the semiconducting layer may contain inorganic particles such as nanotubes or conductive silicon filaments. The lower and upper sublayer can be n-type or p-type and can have doping of the same kind.
In document U.S. Pat. No. 5,629,530 a field effect transistor with a source region, a drain region and a interposed n-type channel region is disclosed. The channel region is provided with a gate electrode that is separated from the channel region by an insulating layer.
An organic thin film transistor is described in document US 2006/0033098 A1. The transistor comprises a substrate, a gate electrode, a gate dielectric layer which covers the entire gate electrode, a source electrode, a drain electrode, an active channel layer and a source interfacial layer. A potential barrier between the source electrode and the active channel layer is reduced by adding an agent into the active channel layer.
The document EP 2 194 582 A1 describes an organic thin film transistor with a substrate, a gate electrode, a source electrode, a drain electrode, an insulator layer, an organic semiconducting layer and a channel control layer that is arranged between the organic semiconducting layer and the insulator layer. The channel control layer includes an amorphous organic compound having an ionization potential of less than 5.8 eV.
In document US 2003/0092232 A1 a further field effect transistor is disclosed.